1) Field of the Invention
This invention relates to a process of resin transfer molding (RTM) used in combination with honeycomb core material and a heat-expandable, foamable material which is heated and expanded to fill the cells of the honeycomb core material. This invention also relates to the strong, lightweight products made thereby. More particularly, the process is for making products through the use of resin transfer molding (RTM) wherein the final product includes a unit of honeycomb core material having cells filled with a foam material (i.e., the RTM resin has been excluded from the cells of the honeycomb core). The strong, lightweight products made by this process are useful in many applications, for example, as aircraft parts.
2) Description of the Background Art
Resin transfer molding (RTM) allows the economical manufacture of high quality composites. Dry fibers, which may have the form of continuous strand mat, unidirectional, woven or knitted preforms, are placed in a closed mold and resin is then introduced into the mold under external pressure or vacuum. The resin cures under the action of its own exotherm, or heat may be applied to the mold to complete the curing process. Early applications of the resin transfer molding technique employed unsaturated polyester resins. Polyester and vinyl ester resins are used in resin-transfer-molded consumer products, pipes, pressure vessels, and automotive applications. Epoxy resins have also been developed for resin transfer molding of high quality, high fiber volume fraction composite components in electronic and aerospace applications.
Resin transfer molding is a process where the resin system is transferred at low viscosities and low pressures into a closed mold die containing a preform of dry fibers. The RTM process can be used to produce low cost composite parts that are complex in shape. These parts typically require continuous fiber reinforcement along with inside mold line and outside mold line controlled surfaces. It is the placement of continuous fiber reinforcements in large structures that sets RTM apart from other liquid molding processes.
For five decades, resin transfer molding has been used for applications suitable to consumer product markets. However, in the last decade through the development of high-strength resin systems and more advanced pumping systems, RTM has advanced to new levels. These recent developments have promoted this technology as a practical manufacturing option for high-strength composite designs, particularly in the aerospace industry.
The following are some of the fundamental advantages of the resin transfer molding process: (1) complex shapes (detail integration); (2) low part variability (product of the mold); (3) good surface finish; (4) 55 to 70% by weight fiber/resin ratio control; (5) eliminates autoclave cycle; (6) low material costs; (7) minimal training costs; (8) low capital investment costs; (9) low worker exposure; and, (10) bushings and inserts can be molded in.
In the aerospace industry, the most visible advantage to this molding process lies in its ability to make complex shapes, that is, to combine multiple, detailed components into one configuration. For example, many traditional designs consist of many individual details that are combined as a subassembly. These subassemblies usually require labor-intensive shimming, bonding, mechanical fastening, and sealing. Consequently, these subassemblies demonstrate high part-to-part variability due to tolerance buildup.
Individual components are integrated into one item with resin transfer molding. Therefore, the part-to-part variation is low because the parts are a product of the mold.
Aerodynamic, decorative finish and controlled fit-up surfaces are typical part characteristics in the aerospace industry. These high-quality surface-finish characteristics are ideal for RTM. Therefore, being a product of the mold makes the surface quality of the part comparable to that of the tool's surface.
Another advantage of RTM is the control of the reinforcement/resin ratio, which is typically 55 to 70% fiber by weight. This produces parts that are lightweight and high in strength.
Because the method of heat transfer is integrated into the mold die, the need for an autoclave is eliminated. Therefore, no autoclave costs are incurred, no size limitations are inherent, and no staging issues occur.
In terms of raw material costs, RTM offers cost savings by using bulk materials like broad goods. Because dry goods are less expensive than preimpregnated materials, a savings can be associated with the cost of the wasted material during the ply-knitting operation. Also, bulk materials do not need special handling requirements such as freezer storage.
The basic injection operation of RTM is straight-forward and easily learned. Hence, minimal training is required to bring operators on line. On the other hand, in making preforms the level of operator skill and training is dependent on the method of preforming that is used.
The initial capital investment costs of RTM are low when compared with the many other molding processes. The most elementary approach to RTM can be achieved using a pressure pot, an oven, and a vacuum source. A variety of commercially-available equipment can be used to advance the process in many areas.
In most cases, RTM materials can be used with minimal chemical exposure to workers and their environment. Many high-performance resin systems are stable and release low volatiles. Since RTM is processed within a closed system, workers are exposed to the resin only when loading the dispensing equipment.
Bushings and inserts can be incorporated into the preform and injected in-place to eliminate some higher level assembly. Special considerations, however, must be made in the design and fabrication of the mold die (i.e., value added vs. tool cost).
Some of the limitations of RTM include: (1) higher tool cost; (2) design changes can be costly (tooling costs); (3) cost of advanced preforming architecture; (4) cost of custom resin systems; and, (5) tool handling challenges (size and weight of tools).
Due to the high quality of the mold and inherent complexity, tooling is expensive. Parts with complex configurations have costly multi-piece, break-down tooling.
Design changes can be costly when modifying complex multi-piece molds. Even a simple design change can result in extensive rework or tool remake.
The cost of advance preforming architecture can be high due to slow labor-intensive processes.
The resin systems must meet design and process parameters that may be difficult to combine. For example, design criteria such as mechanical test values or flammability values must coincide with the process criteria such as pot life, viscosity, worker exposure, and cure time. Resin tougheners, in general, cannot be added because the preform acts as a prime filter entrapping these materials at the point of induction.
One of the benefits of RTM is the ability to manufacture large parts. However, it can also be a major limitation because the tools are large and heavy. Large and massive molds have special handling needs that can include cranes, trunnions, and fork lifts.
The special problem involved in using resin transfer molding to make products that include a unit of honeycomb core material is the exclusion of the resin from the cells of the honeycomb core material. If the honeycomb core cells are not isolated from the resin being injected into the mold, the cells of the honeycomb core will fill with resin and a very heavy product will be the result.